The present invention relates to a filtering process for dehazing a dewaxed lubricating oil base stock. Lube base oils are normally prepared from crude oil distillates and residua or synthetic oils using a series of upgrading steps, which may include hydrocracking or solvent extraction to remove heteroatoms and aromatics and to increase the viscosity index of the base oil; dewaxing to remove wax; and a finishing step for stabilizing the product against oxidation and floc and color formation.
Conventional methods for removing wax from a lube stock include solvent dewaxing and catalytic dewaxing. The degree of dewaxing during one of these dewaxing processes is generally determined by the desired product pour point, where the pour point is a measurement, expressed as a temperature, at which the sample will begin to flow under carefully controlled conditions. Pour point may be determined by, for example, ASTM D5950-96. The cloud point of a lube base oil is complementary to the pour point, and is expressed as a temperature at which a lube oil sample begins to develop a haze under carefully specified conditions. Cloud point may be determined by, for example, ASTM D5773-95. Generally, both the pour point and the cloud point are decreased during dewaxing.
Some lube oil streams, particularly heavy streams such as bright stock, contain naturally-occurring haze precursors that are more difficult to remove by conventional dewaxing than are the paraffinic waxes which predominate in lower boiling waxy streams. If present in sufficient quantities, the haze precursors form a haze in the base oil at ambient (or lower) temperatures, particularly if the base oil is allowed to stand at the low temperature for some time, e.g. overnight. The base oil may develop a hazy appearance even after being dewaxed to a low pour point, e.g. less than xe2x88x925xc2x0 C. Conversely, the haze generally disappears when the base oil is heated slightly, e.g. to a temperature of 80xc2x0 F. or above. The haze will generally be the color of the base oil in which it forms, and is usually white when present in otherwise colorless oil. Haze precursors which give rise to the hazy appearance have significant paraffinic character, some with cyclic components having a long paraffin-like tail. As such, these haze precursors are expected to have substantially different molecular structures than do the color bodies and heteroatom molecules removed by conventional clay filtering for oil stabilization. The presence or absence of a visual haze may be determined using the clear-and-bright standard of ASTM D-4176-93 (Reapproved 10 1997). The haze may also be quantified by measure of clarity.
While the haze generally has little or no effect on the performance of the base oil as a lubricating oil base stock, its presence suggests degraded visual quality and low temperature performance. A method of reducing the haze tendency of a base oil is desired.
The present invention is directed to a process for removing a substantial portion of the haze precursors from a lube stock, and more specifically from a dewaxed lube stock, with little or no reduction in lube stock yield. Accordingly, the present invention provides a process for producing an improved lube oil with a reduced tendency to form a haze after standing at ambient temperatures, the process comprising contacting a dewaxed lube stock with a solid sorbent for a time and at conditions sufficient to produce a dehazed base oil having a reduced cloud point relative, to that of the dewaxed lube stock.
Unlike conventional dewaxing processes, the present filtering process reduces the cloud point of the dewaxed lube stock with little or no effect on the yield of lube base oil. The yield of lube stock based on the weight of dewaxed lube stock feed to the sorbent bed is greater than about 95%, and preferably greater than about 98%. Yields of up to 100% can be expected in some cases during steady state operation. A lube stock which is a preferred feedstock for the present process has a low pour point, typically less than xe2x88x925xc2x0 C., though the pour point may be as low as xe2x88x9240xc2x0 C. and lower, and a cloud/pour point delta of greater than 10xc2x0 C., and preferably greater than 15xc2x0 C. The cloud point of the feedstock is above xe2x88x925xc2x0 C., generally above 0xc2x0 C., and may be as high as 30xc2x0 C. or higher. In the process, the cloud point of the dehazed base oil is reduced relative to the dewaxed oil feedstock to the process. Preferably, the cloud point of the dehazed product from the process is less than 0xc2x0 C., and more preferably less than xe2x88x925xc2x0 C.
Preferred adsorption condition for removing the haze includes a temperature in the range of 1 5xc2x0 C. (60xc2x0 F.) and 60xc2x0 C. (140xc2x0 F.), and a flow rate of hazy oil of between 0.01 hfxe2x88x921 and 10 hrxe2x88x921.
Among other factors, the present invention is based in part on the discovery that certain heavy oil streams develop a visual haze on standing, even when they have been dewaxed to a target pour point using conventional methods. The haze occurs in certain oils having a cloud point above xe2x88x925xc2x0 C., and more often above 0xc2x0 C. or +5xc2x0 C., with a cloud/pour point delta of greater than 10xc2x0 C. The present invention is further based on the surprising discovery that the haze precursors which give rise to the visual haze in these oils may be effectively removed by passing the oil through a bed of solid acidic adsorbent particles. Wax-like molecules which are expected to substantially contribute to the hazy characteristic of such a dewaxed oil would not necessarily be expected to readily and preferentially adsorb on an acid adsorbent. The adsorption process for removing the haze precursors reduces the cloud point of the treated oil while maintaining yields of low cloud point oil which approach 100%.